Reduction of nitric oxide using catalysts pretreated with selenium

ABSTRACT

Base metal catalysts are pretreated with selenium, sulfur or sulfur compounds. Subsequently, a gaseous stream comprising nitric oxide, oxygen and ammonia is passed over the pretreated catalysts. The nitric oxide is reduced to nitrogen and no nitrous oxide is formed.

This is a division of application Ser. No. 657,541 filed Feb. 12, 1976.

BACKGROUND OF THE INVENTION

Nitrogen oxides, particularly nitric oxide, are undesirable products of reaction which result when carbonaceous fuels are burned such as in power plant operations.

Various techniques have been proposed for removing nitric oxides from gaseous streams to prevent pollution of the atmosphere, such as absorption, scrubbing and catalytic conversion.

Catalytic reduction of nitric oxides with ammonia or hydrogen in the presence of nickel and oxides of iron and chromium has been proposed (U.S. Pat. No. 2,381,696; U.S. Pat. No. 3,008,796; and German Pat. No. 1,259,298). The reaction is exothermic and control of the temperature in the catalyst bed is difficult, so that combustion of the ammonia is likely to occur.

Removal of nitric oxides from tail gas streams of nitric acid plants has been attempted by reaction with ammonia, hydrogen, or methane over a catalyst consisting of a supported metal of the platinum group. Anderson et al, Ind. Eng. Chem. Vol. 53, p. 199 (1961); and Adlhart et al, Chem. Eng. Progra. Vol. 67, p. 73-78 (1971). With this method there has been difficulty with control of the exothermic reaction, which results in pressure surges and overheating of the reactor. Also, in some instances, hydrogen cyanide is produced as a by-product.

In power plant emissions, the gaseous effluent typically contains as the major source of pollutants suflur oxides or sulfur dioxide and nitric oxides. It has been found possible to separate the sulfur dioxide from the effluent and to treat the sulfur dioxide separately. This results in an effluent primarily containing sulfur dioxide as less than 2000 ppm, nitric oxide, oxygen and nitrogen and water vapor.

The prior art methods for catalytically reducing nitric oxide with ammonia as a reducing gas experience problems with the temperature of operation required to maintain the efficiency of the catalyst employed, deterioration of the catalyst, controlling exothermic reactions and preventing the formation of by-products which are pollutants, particularly nitrous oxide.

The use of a base metal catalyst to reduce nitric oxide to nitrogen with ammonia in the presence of oxygen and sulfur dioxide has been suggested, German Pat. No. 1,259,298. However, the catalyst life is limited and no controls are provided for the prevention of the formation of nitrous oxide and the exothermic reaction is difficult to control. Further, in similar component systems for the reduction of nitric oxide to nitrogen with ammonia, the use of a copper promoted catalyst on a catalytic support such as alumina, silica or diatomacous earth is suggested, U.S. Pat. No. 3,008,796. The reaction rates are not such that such a process would be considered economically possible for the treatment of a gaseous stream such as from a power plant emission. The elimination or inhibition of the formation of undesirable by-products is not controlled.

SUMMARY OF THE INVENTION

The present invention is broadly directed to the catalytic reduction of nitric oxide to nitrogen with ammonia as a reductant. More particularly, the invention provides a high percent conversion of nitric oxide to nitrogen with ammonia while avoiding or minimizing the formation of undesirable by-products such as nitrous oxide. The invention is directed to the pretreatment of a base metal catalyst selected from the group consisting essentially of copper, iron, chromium, nickel, molybdenum, cobalt, vanadium, the lanthanides and the antinides or any combinations thereof with a nonmetallic element selected from Group VI A of the periodic system.

In a preferred embodiment, the base metal catalysts copper, vanadium and iron either alone or any combination thereof are pretreated with a sulfur compound and/or selenium. The treated catalyst is then employed for the reduction of nitric oxide to nitrogen in a component system of ammonia, oxygen and an inert gas.

In the catalytic reduction of nitric oxide to nitrogen in a multi-component system of nitric oxide, oxygen, ammonia and an inert gas several reactions are believed to occur. The more important reactions are:

    6NO + 4NH.sub.3 → 5N.sub.2 + 6H.sub.2 O

    16no + 4nh.sub.3 → 10n.sub.2 o + 6h.sub.2 o

    3o.sub.2 + 4nh.sub.3 → 2n.sub.2 + 6h.sub.2 o

    4o.sub.2 + 4nh.sub.3 → 2n.sub.2 o + 6h.sub.2 o

    5o.sub.2 + 4nh.sub.3 → 4no + 6h.sub.2 o

in a system of nitric oxide and oxygen with ammonia, to reduce the nitric oxide with or without sulfur dioxide added, using a noble metal catalyst, both nitrogen and nitrous oxide are formed.

In a system of nitric oxide, oxygen and a controlled amount of sulfur dioxide using a base metal catalyst, the nitric oxide is reduced to nitrogen with substantially no nitrous oxide formation. Where the amount of sulfur dioxide in the system is unknown, either no sulfur dioxide or no controlled amount of sulfur dioxide, both nitrogen and nitrous oxide are formed.

In the present invention, base metal catalysts are pretreated and for a system of nitric oxide and oxygen with ammonia using a base metal catalyst, substantially no nitrous oxide is formed. This pretreatment eliminates the necessity of controlling the amount of sulfur dioxide, if any, in the system to eliminate the undesirable by-product nitrous oxide when nitric oxide is reduced to nitrogen with ammonia using a base metal catalyst.

The method of the invention broadly comprises pretreating a base metal catalyst from the group consisting essentially of ferric oxides, vanadium oxides and copper oxides by contacting the catalysts with a pretreatment stream comprising a vaporizable sulfur compound such as dimethyl sulfide, hydrogen sulfide, sulfur dioxide, carbon disulfide or elemental sulfur; or selenium to impregnate the catalyst. Ammonia is blended with a gaseous stream comprising nitric oxide and oxygen to form a blended stream which contacts the pretreated catalyst. The nitric oxide is reduced to nitrogen with substantially no nitrous oxide formation. To promote the formation of metal sulfides on the catalyst or the deposition of sulfur, the pretreatment stream preferably comprises H₂ or NH₃. The pretreatment step is preferably conducted at temperatures between about 400° F. to 900° F. The time of pretreatment or exposure will depend upon the temperature used, and is generally between about 2 hours to 24 hours, preferably between 2 hours to 10 hours, the shorter duration at the higher temperatures. The composition of vaporizable sulfur compound or selenium compound in the pretreatment stream may be 0.5% to 10%, preferably 1% to 2%.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a process flow diagram of the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

A power plant, 800 mw capacity, is shown at 10, and emits a flue gas stream at between about 200° F. to 1,000° F., preferably between 300° F. to 700° F., say for example at 500° F. and at a rate of approximately 200 × 10⁶ cubic feet per hour. A representative composition of the flue gas is set forth below in Table I, it being understood that the composition will vary depending on operating conditions and the type of fuel being consumed.

                  TABLE I                                                          ______________________________________                                         Comp. of Flue Gas                                                              Comp.        Vol. %         lb/hr                                              ______________________________________                                         CO.sub.2     14.5           1.512 × 10.sup.6                             O.sub.2      3.0            2.275 × 10.sup.5                             SO.sub.2     0.2            3.039 × 10.sup.4                             NO.sub.x     0.075          5.329 × 10.sup.3                             Fly Ash      0.2            --                                                 The remainder is comprised of N.sub.2 & H.sub.2 O.                             ______________________________________                                    

The stream is discharged from the power plant 10 having the above composition and is introduced to a precipitator 12 where the approximately 95% of the fly ash is removed. The stream is discharged from the precipitator 12 less the removed fly ash, and ammonia from a source 14 is blended as a reductant gas with the stream to form a blended stream. This blended stream flows to a manifold 16 where it is introduced into a catalytic reactor 20 through a plurality of inlets 18a-d. The ammonia flow rate is dependant on the ammonia-nitric oxide ratio. For this example, Table II lists a range of mole ratios and the associated amount of ammonia available for the subsequent catalytic reaction.

                  TABLE II                                                         ______________________________________                                         NH.sub.3 /NO mole ratio                                                                             lb NH.sub.3 /hr.                                          ______________________________________                                         0.7                  2115.5                                                    0.8                  2417.7                                                    0.9                  2720.0                                                    1.0                  3022.1                                                    ______________________________________                                    

The catalytic reactor comprises a plurality of catalytic beds 22a-d. The streams introduced flow through the catalytic beds where the following reaction primarily occurs.

    6NO + 4NH.sub.4 → 5N.sub.2 + 6H.sub.2 O

the catalyst volume is dependent upon the space velocity. Table III sets forth the cubic footage requirements of the catalytic bed in reference to the space velocity.

                  TABLE III                                                        ______________________________________                                         Space velocity                                                                 hr.sup.-1 reactor    Catalyst                                                  conditions           vol., ft.sup.3                                            ______________________________________                                         25,000               8376.9                                                    50,000               4188.5                                                    75,000               2792.3                                                    100,000              2094.2                                                    ______________________________________                                    

The catalyst employed in this particular embodiment is 10% V₂ O₅ on alumina such as available from Harshaw Chemical and designated VO301, which has been pretreated. The percent reduction of nitric oxide in the blended stream under the conditions set forth herein exceeds 80% and may be approximately 100% with no nitrous oxide formation. A representative composition discharged from the catalytic reactor 20 through outlets 24a-d and through manifold 26 is set forth in Table IV.

                  TABLE IV                                                         ______________________________________                                         Comp. of Reactor Exit Gas                                                      (after electrostatic precipitation)                                            Comp.                Vol. %                                                    ______________________________________                                         CO.sub.2             14.5                                                      O.sub.2              2.9                                                       SO.sub.2             0.2                                                       NO.sub.x             0.020                                                     Fly Ash              0.01                                                      ______________________________________                                          p This reduced stream at between about 300° to 700° F., say      for example 500° F., is introduced into a heat exchanger 18 where      it is cooled by incoming air to about 350° F. It is then discharged      to the stack by a conventional fan 30.

The V₂ O₅ on alumina is pretreated to ensure that there is substantially no nitrous oxide formed, whether or not there is sulfur dioxide present in the stream. The catalyst is contacted with a stream of 2% dimethyl sulfide and 2% hydrogen in helium at a temperature of between about 500° to 700° F., say for example 600° F., for a period of between about 4 to 8 hours, say for example 6 hours. After pretreatment, the catalyst is placed on supports for the beds 22a-d.

Other base metal catalysts which may be similarly pretreated are copper, iron, chromium, nickel, molybdenum, cobalt, or appropriate combinations thereof, normally supported on a high surface area material such as alumina, silica alumina, or zeolites.

In the pretreatment of the catalyst, other suitable compounds which may be used include hydrogen sulfide, sulfur dioxide, carbon disulfide or elemental sulfur; or selenium at operating conditions similar to those set forth above.

In an alternative embodiment, the invention may be utilized for NO removal from the exhaust of a turbine generator employing equipment functionally equivalent to that shown in the drawing. A turbine generator with a capacity of about 20 mw discharges an exhaust of about 30 × 10⁵ cubic feet per hour. A representative composition of the exhaust is set forth below in Table V, it being understood that the composition will vary depending upon operating conditions and the type of fuel being consumed.

                  TABLE V                                                          ______________________________________                                         Comp. of Exhaust Gas                                                           Composition  Vol. Frac.     lb/hr                                              ______________________________________                                         O.sub.2       .16           16.42 × 10.sup.6                             SO.sub.x      28 × 10.sup.-6                                                                         57.30                                              NO           114 × 10.sup.-6                                                                         107.6                                              CO           5 × 10.sup.-6                                                                           4.397                                              ______________________________________                                    

The stream discharged from the generator, having the above composition, is introduced into a catalytic reactor substantially identical to that shown in the drawing and described in reference to the preferred embodiment of the invention. More particularly, the catalytic reactor includes a plurality of catalytic beds in a column-like configuration disposed in a plurality of zones. A reductant gas, more particularly, ammonia, is blended with the exhaust gas stream and introduced through a manifold to a catalytic reactor at a temperature of between about 500° F. to 900° F., say for example 800° F. The ammonia-nitric oxide molar ratio may vary between 0.7 to 1.0. The following Table VI lists the mole ratios and required amount of ammonia necessary for the subsequent catalytic reaction.

                  TABLE VI                                                         ______________________________________                                         NH.sub.3 /NO                                                                   Mole Ratio           lb. NH.sub.3 /hr                                          ______________________________________                                         0.7                  42.72                                                     0.8                  48.82                                                     0.9                  54.93                                                     1.0                  61.03                                                     ______________________________________                                    

The total catalyst volume is dependent upon the space velocity. The following Table VII sets forth the cubic footage requirements of the catalyst in reference to space velocity, which varies between about 25,000 hr⁻¹ to 100,000 hr-1.

                  TABLE VII                                                        ______________________________________                                         Space Vel. Hr.sup.-1 Catalyst Vol.                                             Reactor Cond.        Ft..sup.3                                                 ______________________________________                                         25,000               1257.                                                     50,000               628.4                                                     75,000               418.9                                                     100,000              314.2                                                     ______________________________________                                    

The catalyst employed in this alternative embodiment is V₂ O₅ supported on alumina, which is pretreated as described in the preferred embodiment. The percent reduction of nitric oxide under the conditions set forth herein is substantially 100% with substantially no nitrous oxide formation. A representative composition discharged from the catalytic reactor is set forth below in Table VIII.

                  TABLE VIII                                                       ______________________________________                                                  Comp. of Reactor Exit Gas                                             Comp.                Vol. Frac.                                                ______________________________________                                         O.sub.2              .15                                                       SO.sub.x             28 × 10.sup.-6                                      NO                   50 × 10.sup.-6                                      CO                    5 × 10.sup.-6                                      ______________________________________                                    

The following examples illustrate the suitability of the catalyst compositions when used in the inventive process.

EXAMPLE 1

Samples of six commercially available catalysts were used as received to reduce nitric oxide by ammonia in the absence of sulfur dioxide, as set forth in Table IX below.

                  TABLE IX                                                         ______________________________________                                         Catalyst Type  Manufacturer Identification                                     ______________________________________                                         10% CuO on alumina                                                                            Harshaw      CuO803                                             Cr promoted iron oxide                                                                        Girdler      G3A                                                Copper chromite                                                                               Girdler      G13                                                3% Pt on alumina                                                                              Matthey Bishop                                                                              MB30                                               10% V.sub.2 O.sub.5 on alumina                                                                Harshaw      VO301                                              10% V.sub.2 O.sub.5 on silica                                                                 Harshaw      VO701                                               alumina                                                                       ______________________________________                                    

Approximately 3 grams of each catalyst was changed to individual 1/4 inch diameter aluminum reactors and placed in a furnace such as a Lindberg Heavi-Duty furnace. A feed mixture comprising approximately 520 ppm NH₃, 600 ppm NO, 5000 ppm O₂, and the balance He was passed over these catalysts at a space velocity of 380 std. cc/gm-min. The results are set forth in Table X.

By comparison to the inlet NO level, it can be seen that substantial quantities of NO have been converted, but that the bulk of it has been converted to N₂ O, an undesirable by-product, rather than to N₂, the desired product.

EXAMPLE 2

The catalysts of Example 1 were tested in a similar fashion, except approximately 2000 ppm of sulfur dioxide was added to the feed mixture of Example 1. The results obtained are set forth in Table XI. It can be seen from the table that the addition of sulfur dioxide to the feed of the non-noble metal catalysts has reduced the undesirable formation of N₂ O to zero, with N₂ being the only reaction product in these cases. The addition of sulfur dioxide did not inhibit the formation by N₂ O for the platinum catalyst however.

                  TABLE X                                                          ______________________________________                                                                PPM                                                                            Product Gas                                                                    Composition                                             Catalyst Type     Temp.,° F                                                                          NO      N.sub.2 O                                 ______________________________________                                         3% Pt on alumina  431        11      392                                       3% Pt on alumina  457        10      405                                       3% Pt on alumina  506        46      374                                       3% Pt on alumina  557        79      353                                       10% CuO on alumina                                                                               430        79      119                                       10% CuO on alumina                                                                               457        81      153                                       10% CuO on alumina                                                                               507        119     252                                       10% CuO on alumina                                                                               556        139     331                                       Copper chromite   430        181     108                                       Copper chromite   458        149     131                                       Copper chromite   507        187     194                                       Copper chromite   557        258     275                                       Cr promoted iron oxide                                                                           433        175     153                                       Cr promoted iron oxide                                                                           460        203     267                                       Cr promoted iron oxide                                                                           509        219     297                                       Cr promoted iron oxide                                                                           559        220     301                                       10% V.sub.2 O.sub.5 on alumina                                                                   432        44       55                                       10% V.sub.2 O.sub.5 on alumina                                                                   459        59       86                                       10% V.sub.2 O.sub.5 on alumina                                                                   508        70       42                                       10% V.sub.2 O.sub.5 on alumina                                                                   559        222     322                                       10% V.sub.2 O.sub.5 on silica alumina                                                            433        220      59                                       10% V.sub.2 O.sub.5 on silica alumina                                                            460        140     106                                       10% V.sub.2 O.sub.5 on silica alumina                                                            508        83       93                                       10% V.sub.2 O.sub.5 on silica alumina                                                            560        197     279                                       ______________________________________                                    

                  TABLE XI                                                         ______________________________________                                                                    Down-    Concen-                                                      Temp.    stream   tration                                    Catalyst Type     ° F                                                                              NO       N.sub.2 O                                  ______________________________________                                         3% Pt on alumina  457      165      419                                        3% Pt on alumina  506      131      404                                        3% Pt on alumina  553      143      449                                        10% CuO on alumina                                                                               455      455      0                                          10% CuO on alumina                                                                               505      342      0                                          10% CuO on alumina                                                                               557      91       0                                          Copper chromite   455      534      0                                          Copper chromite   506      477      0                                          Copper chromite   553      415      0                                          Cr promoted iron oxide                                                                           457      470      0                                          Cr promoted iron oxide                                                                           507      273      0                                          Cr promoted iron oxide                                                                           559      29       0                                          10% V.sub.2 O.sub.5 on alumina                                                                   458      63       0                                          10% V.sub.2 O.sub.5 on alumina                                                                   508      0        0                                          10% V.sub.2 O.sub.5 on alumina                                                                   560      0        0                                          10% V.sub.2 O.sub.5 on silica alumina                                                            459      155      0                                          10% V.sub.2 O.sub.5 on silica alumina                                                            510      19       0                                          10% V.sub.2 O.sub.5 on silica alumina                                                            561      0        0                                          ______________________________________                                    

EXAMPLE 3

The catalysts of Example 1 were exposed to a stream of 2% dimethyl sulfide and 2% H₂ in helium for 6 hours at 600° F., and were tested at the conditions of Example 1 with no sulfur dioxide added to the feed to the reactors. Substantially no production of N₂ O was observed for the non-noble metal catalysts, with the only reaction product being N₂ and the downstream concentration of NO and N₂ O being substantially the same as in Table XI. This pretreatment with dimethyl sulfide did not inhibit N₂ O formation with the platinum catalyst.

By pretreating the catalysts with the sulfur or selenium compounds, nitrous oxide formation is prevented whether or not sulfur dioxide is present in the stream containing the nitric oxide. My invention eliminates the requirement of controlling the amount of SO₂ remaining as a flue gas stream for the reduction of nitric oxide without nitrous oxide formation. 

Having described our invention, what we claim is:
 1. A method for the catalytic reduction of nitric oxide which includesa. blending ammonia with a gaseous stream comprising nitric oxide and oxygen, the ammonia added in an amount sufficient to react with the total amount of nitric oxide in a stream to form a blended stream; b. placing the blended stream of step (a) in catalytic contact with a pretreated base metal catalyst at a temperature of between about 300° F. to 700° F. to reduce the nitric oxide to nitrogen while preventing the formation of nitrous oxide, the base metal catalyst selected from the group consisting of copper, vanadium, iron and molybdenum and combinations thereof, which catalyst has been pretreated by contact with a gaseous pretreatment stream containing selenium at a temperature of between about 400° F. to 900° F. to form the pretreated catalyst.
 2. The method of claim 1 wherein the base metal catalyst is selected from the group consiting of copper, vanadium and iron.
 3. The method of claim 1 wherein said pretreatment stream comprises selenium and 2% hydrogen in helium. 